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AD7656 power supply anomaly

Question asked by nmrcarl on Dec 8, 2011
Latest reply on Dec 12, 2011 by nmrcarl

Hi all -


We are using two AD7656 6-channel 18-bit converters on a new product.  We have an unexplained observation on the power supply of the prototype boards.  Here's an overview:


AD7656 power supplies (all voltages nominal)

  VDD = +15

  VSS = -15

  AVcc= +5

  DVcc= +5

  Vdrive = +3.3.


The chips are connected to an Altera MAX II CPLD with VCCIO = +3.3 (nominal).  +3.3V is supplied by a MAX604CSA linear regulator.  One ohm series resistors are connected between the regulator and each chip, for purposes of current measurement. (decoupling caps on both sides of the series R).


Here's the problem:  The actual voltage at Vdrive is almost +4 V, on several boards!  Voltage drops across the series R's indicate 50mA is supplied to the CPLD pins, but the AD7656s are sourcing 45 mA each (i.e. the voltage at AD7676 pin 9 is higher than the regulator output)!  The board actually seems to work, but the power supply anomaly is disturbing.  Any ideas?